Implement out-of-the box panning (including kinetic panning/scrolling) to C++ API.

This item is separated from corresponding QML item (QTMOBILITY-1156) because they likely have different implementation timelines;
C++ proved to have significant binary compatibility challenges: the QGraphicsGeoMap is not the primary virtual base class of QGraphicsItem where it inherits the relevant mouse event handlers from, and furthermore QGraphicsGeoMap does not override these virtuals. This means that adding implementation for these virtuals will break BC of any child classes.

An alternative considered was to implement new graphicsgeomap class. However at the moment just kinetic / panning is not necessarily enough to justify the class, but if more similar value adding opportunities emerge, this should be considered an option.